Transmission

Variant CJD is believed to have been transmitted by consuming infected beef products such as sausages, franks, and hamburgers that contained portions of brain, spinal cord, dorsal root ganglia, retina, intestines, or bone marrow as components. A list of Specified Risk Materials (SRM) has been adopted by the European Union (EU) Scientific Steering Committee as a result of calf assay experiments using various tissues from known infected animals (Table 1; 22).

Prions are extremely resistant to heat, ultraviolet light, ionizing radiation, and common disinfectants that normally inactivate viruses and bacteria, and destroy nucleic acid (2,3,23-25). Contaminated surgical instruments have been shown to retain infectivity following standard hospital sterilization techniques. Transplanted tissues from humans with CJD, including dura mater grafts, extracts from pituitary glands, and corneas, have been demonstrated vehicles of transmission. Strategies to protect the blood supply, including provisions to deplete blood products of leukocytes because these white blood cells may play a role in the transport of ingested abnormal prions to the nervous system, are currently under consideration in the UK (26).

Kuru was transmitted by ingestion and through open cuts or abrasions. Scrapie in sheep has been transmitted through open wounds and new infections have been associated with lambing of normal animals with infected animals. Although the majority of cattle infections resulted from ingestion of contaminated protein feed supplements, studies on potential maternal transmission report a small risk of infection for calves born shortly before or after the onset of clinical signs of the disease in infected dams. Experts indicate this rate is insufficient to perpetuate the epizootic in cattle but will delay the eradication effort (6,27).

Table 1. Levels of TSE Infectivity Associated with Various Ruminant Tissues

Category

Organs

High infectivity

Medium infectivity

Low infectivity No detected infectivity

Bovine brain, eyes, spinal cord and dorsal root ganglia; dura mater, pituitary, skull and bovine vertebral column, lungs ovine/caprine brain, eyes, and spinal cord, dorsad root ganglia and vertebral columns; ovine and caprine spleens, lungs Total intestine from duodenum to rectum, tonsils Bovine and caprine spleen, placenta, uterus, fetal tissue, adrenal, cerebrospinal fluid, lymph nodes

Liver, pancreas, thymus, bone marrow, other bones, nasal mucosa, peripheral nerves Skeletal muscle, heart, kidney, colostrum, milk, discrete adipose tissues, salivary gland, saliva, thyroid, mammary gland, ovary, testis, seminal testes, cartilaginous tissue, skin, hair, blood clot, serum, urine, bile, feces

The persistence of chronic wasting disease of mule deer and elk raises questions about possible methods of lateral transmission between unrelated animals. There is a small risk of transmission to uninfected lambs if they are born in the immediate physical vicinity of infected ewes also giving birth.

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